Continued from above…

Subtle light is not controlled by a neophyte yogi. At first he has to practice steadily until the light appears. When it appears it does so independently of his will power. After some practice, he may link his attention to it. Initially when a yogi tries to link or focus on the inner light or lights, it disappears from view, as if to tease him. This occurs because of the unsteady intellect. To steady the intellect further, he has to perfect the prāņāyāma practice which causes the psyche to become purified by the full expulsion of the apāna de-energizing airs. Bhastrika breath exercises are a good method of causing psychic purification, but there are other prāņāyāmas which achieve this.

A yogi has to practice detachment and must remove himself from all associations, which do not help to accelerate practice, otherwise he cannot be successful.

मनो मयि सु-सयोज्य

देहं तद्-अनुवायुना ।

मद्-धारणानुभावेन

तत्रात्मा यत्र वै मनः ॥१०.२१॥

mano mayi su-saṁyojya

dehaṁ tad-anuvāyunā

mad-dhāraṇānubhāvena

tatrātmā yatra vai manaḥ (10.21)

mano = manah — the mind; mayi — on me; su-samyojya — easily and continually connecting; deham — body; tad = tat — that; anuvāyunā — by the life air that trails; mad = mat – on me; dhāraṇānubhāvena = dhāraṇā — linking the attention with the concentration energy; anubhāvena — by the inner nature; tatrātmā = tatra — there + ātmā — spirit; yatra — wherever; vai — surely; manaḥ — the mind.

Translation

Easily and continually connecting to Me, the mind, the body and the life air that trails the mind, and by linking the attention with the concentration energy, which is operative in the inner nature, and which is focused on Me, the spirit surely goes wherever the mind travels. (10.21)

Commentary:

Difficult as these procedures sound, they are elementary to the practices mentioned before which are the advanced stages of yoga dealing with aspects of atomic, cosmic and spiritual importance. However, a neophyte benefits from these descriptions since he will have to progress through these basic stages after he masters pratyāhar practice.

Instead of saying samyojya, Śrī Krishna added the prefix su, which means easy. Thus this practice applies when the yogi can easily (su) and continually (su-samyojya) perform the 6th stage of yoga, that of dhāraņā. In such practice, it is not that the yogi focuses the energy himself. The yogi energizes his subtle body and the energies within it, become focused of their own accord or alternately, energies from other dimensions begin to penetrate into his energized psyche in a focused way. The yogi then acts to link his will power, his attentive energy, to that focused force. It is very important to understand what dhāraņā is, otherwise one may assume that it means the focusing of one’s will power. It means the linking or joining of one’s attention with a naturally focused force.

Śrī Krishna mentioned the mind, the body and the life force which trails the mind. This means that the yogi must have mastered kundalini yoga, since otherwise the mind will trail the life force instead. It does not matter whether one is a devotee or not, if one has not mastered kundalini yoga, his mind will have to trail his life force. Only those yogis who mastered kundalini chakra to cause it to rise up and out of the base chakra, the mūlādhāra, will have their life force trail the mind or precisely, trail the buddhi organ in the subtle head. They raise the kundalini energy up into the brain either to the back or to the frontal lobe or to the top of the head.

This applies to yogis who mastered the āsana posture and inner body purity by prāņāyāma, because only such persons can make their body obey their will in this manner. Others cannot do it, even if they are devotees of Śrī Krishna. The traveling of the mind is mentioned, with the spirit following the mind. This is because at this stage of practice, the spirit remains behind the buddhi organ as it seeks out higher dimensions. Being behind the buddhi organ, actually means being centralized behind it or being in the centre of it directorially, with it before or on all sides spherically.

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The main thing about those verses is dhyana yoga application or application of samadhi to various supernatural and mystic objects.

Even if there is mention of a physical object, even then it is the subtle part of that physical object.

But you may also find the verses in Chapter 3 of Yoga Sutras, I think, where Patanjali goes over the same process.

The origin Vedic use of tvastr is given on the page below but even here you have to remain in the context of mystic yoga and not bring over any physical meanings into it.

Link

If you require some scripture support better to find one and if there is none then you just have to do that practice as you are and continue with it.

One of the main reasons why I first did the translation and commentary of Bhagavad Gita was to show support for yoga practice, ashtanga yoga, because the Vaishnava authorities had over centuries completely destroyed the support. And without Krishna's support one cannot complete the practice.

Support is needed but you must get it from a valid place not railroad some other text and try to use it for support.

Whosoever is teaching this sun gazing stuff should be responsible to give the required scriptural supports.